1. Field of the Invention
The present invention relates to a telephoto lens with a large relative aperture.
2. Description of the Prior Art
Nowadays there is an increasing demand for large relative aperture telephoto lenses for use in still cameras, particularly for 35 mm still cameras by reducing the size and weight of such lenses to thereby improve their portability and to expand the field of photography. Also, there is a need that a telephoto lens possess the capability of focusing upon an object at a comparatively short distance.
A system hitherto commonly used in altering the focusing position of an ordinary lens from an object at infinity to an object at a short distance is called "total drawing-out system" according to which the entire lens is shifted toward the object side. However, this system is useful only with ordinary lenses, and is not applicable to those telephoto lenses of large relative aperture, for example a focal length of 200 mm or more and an F number of approximately 2.0. Attempts to apply the aforementioned system to a large relative aperture telephoto lens gives rise to the following problems:
(a) in the position set for short distance photography, the various aberrations become unmanageable, and therefore, the minimum distance allowable necessarily becomes a considerably large value;
(b) the whole configuration of the lens becomes large and therefore the total drawing-out mechanism itself becomes large and complicated to the extent that a smooth focusing operation cannot be obtained; and
(c) since the diameter of the diaphragm becomes large, it is difficult to employ a so-called automatic diaphragm mechanism in which the motion of diaphragm blade is linked to the release of the shutter.